Inefficient assembly and intracellular accumulation of antibodies with mutations in V(H) CDR2

We previously described secretion defects in four mutants of the murine anti-phosphocholine Ab, T15. The mutant heavy (H) chains had amino acid replacements in the V(H) complementarity-determining region 2 (HCDR2) and were expressed at normal intracellular levels. Here, the intracellular fate of the secretion-defective mutant heavy chains was investigated. Metabolic labeling demonstrated that the T15 wild-type Ab was secreted within a 4-h chase. In contrast, the mutant H chains accumulated with intracellular t(1/2) values ranging from 10 to 24 h. The mutant H chains were associated with increased levels of the molecular chaperones BiP and GRP94, and remained endoglycosidase H sensitive, suggesting retention in the endoplasmic reticulum. Assembly of the mutant H chains with T15 light (L) chain was arrested at the H₂ and H₂L intermediate stages of the T15 wild-type pathway (H₂ → H₂L → H₂L₂). Even though some assembly with L chain occurred, it was not as a secretion-competent H₂L₂ Ig moiety. The T15 L chains coexpressed with mutant H chains were degraded efficiently except for a minor L chain population with a long t(1/2) that was apparently protected at the H₂L stage. To our knowledge, this is the first study demonstrating that intracellular half-lives of Ig H and L chains can be influenced by somatic mutations in HCDR2.